1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image recording, a laser image forming apparatus is widely used. Such an image forming apparatus includes an optical scanning device, and generally forms a latent image by rotating a photosensitive drum while scanning the drum with a laser beam by a polygon scanner (for example, a polygon mirror) in an axial direction of the drum. In the electrophotographic field, the image forming apparatus is required to achieve high-density imaging to improve image quality and high-speed image output to improve operability.
To achieve both the high-density imaging and the high-speed image output, the polygon scanner can be rotated at a high speed. However, this results in the increase in noise of the polygon scanner and power consumption, and decrease in durability.
As another method of achieving both the high-density imaging and the high-speed image output, multi-beams of a light flux can be emitted from the light source. There are methods to obtain such an multi-beam as follows: (1) a method of combining a plurality of end-surface light-emitting lasers, (2) a method of using a one-dimensional array of end-surface light-emitting lasers, and (3) a method of using a two-dimensional array of a vertical-cavity surface-emitting LASERs (VCELs). Reference may be had to, for example, Japanese Patent Application Laid-open Nos. 2005-250319 and 2004-287292.
According to the method (1), cost can be decreased by using a general-purpose laser. However, it is difficult to stably hold a relative positional relationship between the coupling lens and the lasers emitting a plurality of light beams. Therefore, an interval between adjacent two scanning lines among a plurality of scanning lines formed on a scanning surface (hereinafter, “scanning-line interval”) may become uneven. Further, there is a limit to the number of light sources, and there is also a limit to increase density and speed. According to the method (2), while the scanning line interval can be made uniform, there is an inconvenience that the power consumption of elements increases. When the number of light sources is considerably increased, a deviation amount of beams from the light axis of the optical system becomes large, and the beam quality may be degraded.
On the other hand, according to the method (3), less power consumption is required than that of the end-surface light-emitting laser by about one digit, and more light sources can be two-dimensionally integrated.
Japanese Patent Application Laid-open No. 2005-250319 discloses an image forming apparatus including a light source device including a plurality of sets of light sources, each having a plurality of independently modulatable light emitting points arranged two dimensionally, and a coupling lens that couples dispersion light flux emitted from the light source; an optical scanning device mounted with the light source device; and an image forming apparatus mounted with the optical scanning device.
Japanese Patent Application Laid-open No. 2004-287292 discloses an image forming apparatus mounted with an optical scanning device using a surface light-emitting laser array, and an image forming apparatus mounted with the optical scanning device.
Usually, to suppress the occurrence of uneven concentration of light on an image due to a change of light intensity following a temperature variation and a time change, the optical scanning device monitors the optical amount of the light flux emitted from the light source with a detector such as a photodiode, and performs auto power control (APC) for controlling the output level based on a monitoring result. In this case, in the end-surface light-emitting laser, light fluxes are emitted to two directions of forward and backward. Therefore, when the light flux emitted forward is used for scanning, and also when the light flux emitted backward is used for monitoring, there is no influence in the monitoring result even when the scanning light flux returns to the light source. However, because the surface light-emitting laser emits the light flux to only one direction, the emitted light flux needs to be divided or branched into two, and one of the light fluxes needs to be used for scanning and the other needs to be used for monitoring. In this case, the light flux returning to the light source may influence the monitoring result.
Recent electrophotographic image forming apparatuses, such as a digital copier, a laser printer, a laser plotter, and a laser facsimile machine, or a multifunction product (MFP) that combines any or all of the functions of these, are capable of forming color images at an increasingly high speed, and tandem color-image forming apparatuses having a plurality of (generally, four) photosensitive elements are in widespread use.
However, in the optical scanning device provided in the tandem image forming apparatus, the number of light sources increases correspondingly to photosensitive elements. Therefore, cost increases due to the increase in the number of parts, and a color shift occurs due to a wavelength difference between a plurality of light sources.
Generally, the cost of the light source in the optical scanning device is high because the light source includes a substrate for modulating a semiconductor laser. This high cost is against the cost reduction of the device and miniaturization thereof.
Degradation of the semiconductor laser used as a light source causes failure of the optical scanning device. An increase in the number of light sources may increase the risk of failure, and this is disadvantageous for recycling.
Japanese Patent Application Laid-open No. 2002-23085 discloses an example of a conventional optical scanning device that is designed to avoid increasing the number of light sources in the optical scanning device corresponding to the tandem system. The conventional optical scanning device uses a pyramidal mirror or a sheet mirror to scan different scanning surfaces with light beams from the same light source. With this, while the number of light sources can be decreased, the maximum number of surfaces of a deflection mirror is two, and this is a problem in achieving a high speed.
To solve the above problems in the conventional technology, according to an embodiment of the present invention, a deflector having a polygon mirror, with phases deviated and superimposed at two stages, is used to scan different scanning surfaces with light beams from the same light source. Japanese Patent Application Laid-open No. 2001-83452 discloses a conventional technology related to the embodiment of the present invention. However, this conventional technology has an object of increasing a scanning width, and not scanning different scanning surfaces.
Further, recent optical scanning devices for a color image forming apparatus include an oblique-incidence optical system that inputs (oblique incidence) an light beam to a deflection-reflection surface of an optical deflector at an angle in the sub-scanning direction. An example of such an optical scanning device is described in Japanese Patent Application Laid-open No. 2003-5114. With this oblique-incidence optical system, after a plurality of light beams are deflected/reflected on the deflection-reflection surface, the reflected beams are separated and led by a reflection mirror or the like to corresponding scanning surfaces of photosensitive elements. Angles of the light beams in the sub-scanning directions (angles at which the light beams are obliquely incident to the optical deflector) are set to angles at which each light flux can be separated by the mirror.
However, the oblique-incidence method causes a “scanning line curve”, and an increase of variation between image heights of a beam spot diameter due to “wave aberration degradation”. The occurrence amount of the scanning line curve is different depending on oblique-incidence angles of the light beams in the sub-scanning directions. When latent images formed by respective light beams are developed into toner (visible) images with toner of respective colors and superimposed, color shifts occur. Because light fluxes are incident at oblique angles, the light fluxes are twisted when incident to the scanning lens, resulting in the increase in the wave aberration. Particularly, the optical performance is considerably degraded in the peripheral image heights, and the beam spot diameter increases. As a result, this interrupts the high-image quality.
Japanese Patent Application Laid-open No. H11-14932 discloses, as a method of correcting a “large scanning-line curve” intrinsic to the oblique-incidence method, a method of including “a lens having a lens surface, with an intrinsic inclination of the lens surface in a sub-scanning cross section changed to a main scanning direction to correct a scanning line curve” in the scanning imaging optical system. Japanese Patent Application Laid-open No. H11-38348 discloses a method of including “a correction reflection surface having a reflection surface with an intrinsic inclination of the reflection surface in the sub-scanning cross section changed to a main scanning direction to correct a scanning line curve” in the scanning imaging optical system.
Japanese Patent Application Laid-open No. 2004-70109 has proposed a conventional technology in which a light flux incident at an oblique angle is passed through off-axis of the scanning lens. Scanning lines are adjusted using a surface for changing the aspherical amount of a child line of the scanning lens along the main scanning direction. According to this conventional technology, correction is performed using one scanning lens. While the scanning line curve can be corrected, degradation of the beam spot diameter due to an increase in the wave aberration is not taken into account.
Another problem of the oblique-incidence method is that a large degradation of wave aberration easily occurs at the peripheral image height (near both ends of the scanning line) due to the beam skew. When this wave aberration occurs, a spot diameter of the optical spot becomes large at the peripheral image height. That is, the diameter of beam spots varies between image heights. When this problem cannot be solved, high image quality cannot be achieved. With this conventional technology, a large scanning line curve intrinsic to the oblique-incidence method is satisfactorily corrected. However, the correction of the wave aberration cannot be said satisfactory.
Japanese Patent Application Laid-open No. H10-73778 discloses as a device capable of satisfactorily correcting “scanning line curve and degradation of wave aberration” mentioned above as problems of the oblique-incidence method, an optical scanning device that includes a plurality of rotation asymmetrical lenses in the scanning imaging optical system, in which the bus-line shape connecting between child-line tops of the lens surfaces of the rotation asymmetrical lenses is bended to the sub-scanning direction.
The lens having the “lens surface having the bus-line shape connecting between child-line tops bent to the sub-scanning direction” mentioned above solves the problems by bending the bus line. However, because individual scanning lenses corresponding to the incident light fluxes are necessary, the number of scanning lenses increases when the scanning lenses are applied to the tandem scanning optical system. That is, when a plurality of light fluxes directed to different scanning surfaces are incident to the same lens, the problems can be solved for one light flux by bending the bus-line shape; however, the scanning-line bending and wave aberration cannot be easily decreased for the other light flux.
Because light fluxes have a curvature in the sub-scanning direction, when the light fluxes incident to the same lens are deviated in the sub-scanning direction due to an assembly error, a processing error, and environmental variations, the shape of the scanning-line curve changes because of the influence of the refractive power of the lens in the sub-scanning direction. Thus, color shift cannot be avoided in an color image at the initial time (or at the design time).
In the correction of wave aberration, there is a large change in the skew state of the light flux due to the variation of the incident light flux on the surface having a curvature, and a satisfactory beam-spot diameter cannot be easily obtained stably.
In the conventional technology related to the oblique incidence method described in Japanese Patent Application Laid-open No. 2003-5114, a surface similar to that described in Japanese Patent Application Laid-open No. H10-73778 is used to correct the scanning line curve. However, as explained above, it is difficult to stably obtain a satisfactory beam spot diameter.